Electric submersible pumps (ESP) (also referred to as deep well submersible (DWS) pumps) are especially useful in extracting valuable resources from deep well geological formations. In one example, an ESP can be used to retrieve crude oil or natural gas from significant subterranean depths. In another widely-used example, an ESP provides the motive power to large quantities of water, such as those used in municipal waterworks. ESPs conventionally include a centrifugal pump section and a motor section that are axially aligned with one another and oriented vertically in the well. More particularly, the motor section may be configured to drive one or more pump section stages.
Because ESPs are relatively inaccessible (often completely submerged at distances between about 10 and 700 meters beneath the earth's surface), they must be able to run for extended periods without requiring maintenance. Such extended operating times means that the economic feasibility of a system utilizing such pumps requires dependable, robust componentry. This can frequently be at odds with the need to design more efficient pumping systems as a way to alleviate increasing energy costs and more stringent environmental policies.
One particular motor configuration that has shown promise for high-efficiency operation is based on the use of a permanent magnet rotor, and is known as a permanent magnet synchronous motor (PMSM). Unfortunately, these are more expensive to build than conventional (asynchronous) induction motors, thereby offsetting the gains made possible by the inherent efficiency improvements. More significantly, the peculiar environment associated with submersible motors can hamper the long-term robustness that is needed in order to justify the higher cost of the PMSM; such environmental concerns include (a) requiring the use of a long, thin rotor to fit in limited diameter boreholes or pipes as a way to maximize the power output at a given outer motor diameter, (b) preventing the corrosive effects on the magnets of the motor lubricant or working fluid, and (c) the lengthy meantime between service of ESPs. More particularly, the first concern takes into consideration that the motors (and their respective shafts) used in an ESP define a long, thin profile where the magnetic and rotordynamic forces would tend to cause shaft deflection. Such deflection can undesirably lead to rotor contact with the stator. Likewise, the second concern is not ameliorated by the use of coatings on the magnets, as the prolonged exposure of the magnets to fluids (which are often at elevated temperature) will invariably lead to material degradation. Regarding this third concern, such exposure (and related magnet and motor damage) is inconsistent with the desired long times between service that are needed to make submersible pumps economically viable.
What is needed is a high-efficiency PMSM design for use in a submersible pump system that provides improved protection of the motor's rotor-mounted permanent magnets, as well as enhanced durability for the rotor.